Patients with hyperthyroidism often exhibit signs and symptoms of anxiety, making it reasonable to question the role of thyroid dysfunction in individuals with anxiety disorders. We have shown that patients with panic disorder and patients with social phobia have normal peripheral thyroid indices, normal rates of antithyroid antibodies, and normal TSH responses to TRH. In addition, we have found that patients with panic disorder have normal QKd intervals, a putative measure of thyroid hormone effect. These studies have suggested to us that patients with panic disorder and social phobia are unlikely to exhibit thyroid dysfunction as part of the pathophysiology of either of those disorders. This contrasts with a number of studies which point to the existence of thyroid abnormalities in at least some patients with major depression, and suggests that this may be an area where panic disorder and major depression diverge.